Bioassays and biosensors that detect and quantify biomolecules at ultra-low quantity, with point-of-care settings, are of great need in many fields, including basic medical science, disease control and diagnostics, drug discovery and environment monitoring. Bioassays can be used for disease or pathogen detection based on the principle of specific interactions between biological components Antibody-antigen interaction is one example of a specific interaction between biomolecules which can be used. Some other examples of biological interactions include interactions between oligonucleotides, such as DNA-DNA or RNA-RNA interaction, small-molecule-biomolecule interactions, aptamer-biomolecule interactions, protein interactions, and others.
Magnetic biosensors include giant magnetoresistive (GMR) sensors, magnetic tunnel junction (MTJ) sensors, Hall biosensors or Giant magneto impedance (GMI) biosensors. Magnetic biosensing, which combines the magnetic biosensor and magnetic nanoparticles (MNPs), is a field which has been intensively studied. In existing magnetic biosensing schemes, when the targeted biomolecues are present, they bond to the biologically-functionalized surface of an individual magnetic field sensor or a sensor-array. Functionalized MNPs bond to these targeted biomolecules. The dipole field from the specifically bound magnetized MNPs will change the overall effective magnetic field on the sensing layer of the magnetic biosensor. This causes the change of the magnetization configuration of the magnetic biosensor, thus generating an electrical signal from the biosensor, which can be quantitatively correlated with the number of the MNPs.
Conventionally, as shown in FIGS. 7 and 8, a powerful, externally-applied magnetic field is required and operates as the exclusive source for magnetizing the MNPs. Hence, in conventional magnetic biosensing schemes, the necessity for an external magnetic field generator downplays the promised portability feature for magnetic biosensors and also increases the power consumption of the whole system.
Generating a magnetic field from built-in current lines on the biosensor has also been proposed for magnetizing the MNPs. This can eliminate the usage of an external electromagnet. However, the requirement for large power consumption still exists. The presence of a large current on a sensing chip, such as tens of milliamperes, is typically required to produce a large enough magnetic field for magnetizing the MNPs. Such a large current causes heating effects and may also result in a dielectric break down between the protection layer and the biological sample.